1. Field of the Invention
The present invention relates to a process of regenerating palladium salt catalysts.
2. DESCRIPTION OF PRIOR ART
Palladium salt catalysts have been widely used in catalyzing such reactions as oxidation, dehydrogenation, hydrogenation, isomerization and dimerization.
For example, the palladium salt catalysts are used in reactions of producing an aldehyde from an olefin by the Wacker process, forming a glycol ester for forming vinyl acetate from an olefin and acetic acid, and dimerizing benzene and terephthalates.
It is also known that a biphenyl tetracarboxylic acid ester can be produced through oxidative coupling in the presene of a palladium salt catalyst. Generally, the biphenyl tetracarboxylic acid ester formed in the reaction is in the form of a mixture of both isomers, namely, 3,3',4,4'-biphenyl tetracarboxylic acid tetraalkyl ester and 2,3,3',4'-biphenyl tetracarboxylic acid tetraalkyl ester. In this case, however, if a catalyst containing a palladium salt, a copper salt, and a basic bidentate ligand such as 1,10-phenanthroline or 2,2'-bipyridine is present in the reaction system, the former isomer, which is useful as a starting material for producing polyimide, can be formed selectively. Therefore, the process using the above-mentioned catalyst is quite adequate for commercial productions.
Since palladium is a very expensive metal, it is desirable to recover a palladium salt catalyst for regeneration from the reaction solution after completion of the reaction such as of oxidation, dehydrogenation, hydrogenation, isomerization or dimerization, thereby using repeatedly the palladium metal catalyst.
The recovery and regeneration of a palladium salt catalyst can be performed in the following manner. A palladium salt catalyst is separated from the reaction solution in combination with an organic substance; then, the separated residue is first burnt in order to eliminate the organic substance, and then palladium contained in the resultant ash is regenerated by an activation treatment using an acid.
The regeneration process which involves the burning step has been disclosed, for example, in Japanese Patent Provisional Publication No. 54(1979)-9597, in which a deactivated palladium catalyst, which has been produced by the reaction in a lower aliphatic monocarboxylic acid, is first heated above 450.degree. C. in the presence of air for oxidation and then subjected to treatment in nitrogen gas.
However, if a palladium salt catalyst possessing a great amount of an organic substance (e.g. a palladium salt catalyst separated from an organic reaction solution) is heated directly in an atmosphere containing oxygen such as air, the organic substance on the surface of the palladium salt catalyst begins to burn first, which consequently gives a hard metallic surface to the palladium salt catalyst, and the organic substance being present inside of the palladium catalyst is left unburnt. Therefore, the regeneration efficiency is very low in the process. To increase the efficiency, the firing should be performed over a long period of time. Further, if the burning is incomplete and if a large amount of unburnt carbon remains in the ash, the efficiency of regenerating the palladium salt catalyst is extremely lessened Accordingly, the above-mentioned process is not satisfactory as a commercially employable regeneration process.